I have the same thing and I beleive its wash down from the brake rotors and pads.I have cleaned the wheels once off the car when I had my track tires on and its shown up again after a few weeks and washes.I now intend to keep my wheels bathed in brake dust from enjoying the car,so I do not notice the rust runoff

Rust off the rotors is the likely culprit. Aluminum wheels do not rust in the same way that iron or steel does. Oxidization on aluminum is more blackish, not the orange-looking rust that comes from iron/steel.

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I have accepted a new position at Tire Rack and am no longer in the sales department. gary@tirerack.com is available to assist with M3 questions.

Rust off the rotors is the likely culprit. Aluminum wheels do not rust in the same way that iron or steel does. Oxidization on aluminum is more blackish, not the orange-looking rust that comes from iron/steel.

I should have been more specific...the rust on my M3 is coming from the rotors, not the wheel. I notice it after I wash my car.